Katabatic winds

Katabatic wind (from the Greek: katabaino - to go down) is the generic term for downslope winds flowing from high elevations of mountains, plateaus, and hills down their slopes to the valleys or planes below. Katabatic winds exist in many parts of the World and there are many different names for katabatic winds depending where they are located and how they are formed.

Warm, dry katabatic winds occur on the lee side of a mountain range situated in the path of a depression. Examples for these descending, adiabatically warmed katabatic winds are the
Foehn winds.

Cold and usually dry katabatic winds, like the Bora, result from the downslope gravity flow of cold, dense air. Katabatic flows slumping down from uplands or mountains may be funneled and strengthened by the landscape and are then known as mountain gap wind such as the
Santa Ana, mountain breeze or drainage wind. The gentler katabatic flows of hill slopes produce
frost hollows. Mountain breezes are part of a local wind system. When the mountainside is heated by the Sun the mountain breeze will break down, reverse and blowing upslope. These winds are known as valley wind or anabatic wind.

However, katabatic winds might also reach gale force as an outblowing wind over ice-covered surfaces in Antarctica and Greenland, where the wind may be extremely strong and gusty near the coasts and less severe in mountain regions.

Most katabatic winds (except the Foehn) are more or less the result of air in contact with upper level ground is cooled by radiation, increases in density, and flows downhill and along the valley bottom. For example radiation cooling during nighttime can cause a katabatic flow in the early morning when a pool of cold, high elevation air begins to descend beneath warmer, less dense air. This effect is enhanced during winter over snow covered surfaces and after dry, clear nights. These types of winds can reach velocities of up to 4 meters per second. The rush of cold air down the slopes rapidly decreases the surrounding air temperature as it speeds down the topography to the valleys and planes below it.

Many strong katabatic winds are synoptically triggered or are activated by large scale weather features such as a high pressure system over high elevations, as in Greenland, in California or, for example, the fjords of Norway.